Open scattered light smoke detectors are described, for example, in the international patent application WO 2001/031602 A1 or in the European patent application EP 2 093 733 A1.
The European patent application EP 1 191 496 A1 likewise describes an open scattered light smoke detector. In this, an amplitude modulator in the an open scattered light smoke detector forms, from the electrical signal for the transmitter actuation, a sequence of pulses, and in doing so effects an amplitude modulation. In its simplest form, this is a switch, so that a sequence of periodic light pulses is generated in the light transmitter, and then a dark period is again effected, and this alternately in a cycle which is prescribed by the amplitude modulator. A processor then evaluates the received signals by comparison with these transmitted signals, which the amplitude modulator transmits directly to the processor. By this means, the processor is in a position, on the one hand, to make a determination of how far away an item is by reference to the phase shift between the emitted and received pulses and, on the other hand, to check whether the item is a smoke cloud or an object. In the case of a solid object, the width of the received pulse remains unchanged by comparison with the width of the emitted pulse (FIG. 6). On the other hand, if the emitted pulse sequence meets a smoke cloud, then the pulses in the emitted sequence of pulses scatter, and there is pulse dispersion, because there are many centers of scattering in the smoke cloud (FIG. 7). The width of the pulses received is then a measure of whether smoke is present or not.